Heretofore, there have been proposed various boost DC/DC converters (for example, Japanese Patent Laid-Open Publication Nos. 2003-111390 and 2003-216255). FIG. 37 hereof shows a fundamental circuit structure in the conventional boost DC/DC converter disclosed in one of the above-identified publications. The boost DC/DC converter shown in FIG. 37 is in the form of electric circuitry capable of variable voltage boosting. This boost DC/DC converter 100 is constructed using a single coil (i.e., inductor or inductance element) 101.
More specifically, the DC/DC converter 100 of FIG. 37 includes an input-side smoothing capacitor 102, the coil 101, a switching element 103, a diode 104, and an output-side smoothing capacitor 105. The input-side smoothing capacitor 102 is connected between a common reference terminal (ground terminal) 106 and an input terminal 107, and the output-side smoothing capacitor 105 is connected between the common reference terminal 106 and an output terminal 108. Series circuit of the coil 101 and diode 104 is connected between the input terminal 107 and the output terminal 108. The switching element 103 is connected between an intermediate point 109 between the coil 101 and diode 104 and the common reference terminal 106. The switching element 103 is a transistor having bipolar characteristics. Collector of the switching element 103 is connected to the intermediate point 109, and an emitter of the switching element 103 is connected to the common reference terminal 106. Further, a gate of the switching element 103 is connected to a not-shown control device so that a gate signal SG101 is supplied thereto from the control device. The switching element 103 is turned on/off on the basis of the supplied gate signal SG101.
Operation of the DC/DC converter 100 is briefed below. At an initial stage, the input-side smoothing capacitor 102 is charged with an input voltage applied to the input terminal 107 in such a manner that a voltage between its two terminals (inter-terminal voltage) agrees with the input voltage. Once the switching element 103 is turned on, a current flows, on the basis of an electric charge accumulated in the input-side smoothing capacitor 102, to the ground via the coil 101 and switching element 103. During that time, the coil 101 is energized or excited and hence magnetic energy is accumulated in the coil 101. Once the switching element 103 is turned off, an induced voltage based on the magnetic energy accumulated in the coil 101 is convoluted with the voltage of the input-side smoothing capacitor 102, so that a voltage greater than the input voltage is produced and the thus-produced voltage supplies an output current lout to the output-side smoothing capacitor 105 via the diode 104. Adjusting the ON/OFF duty cycle of the switching element 103 can provide a desired output voltage within a predetermined range. In this manner, a variable boost DC/DC converter can be provided.
However, in the conventional boost DC/DC converter shown in FIG. 37, the coil 101 has to have a very great size and weight because the voltage boosting is effected by temporarily storing magnetic energy in the single coil 101. In addition, the boost DC/DC converter would present the inconvenience that the operating efficiency lowers as the voltage boosting ratio is increased.
Further, in recent years, boost DC/DC converters have been proposed which are designed to reduce a core loss and copper loss (see, for example, Wei Wen and Yim-Shu Lee, “A Two-Channel Interleaved Boost Converter with Reduced Core Loss and Copper Loss” exhibited in the 35th-year IEEE Power Electronics Special Conference, Jun. 22, 2004). These boost DC/DC converters each employ an integrate magnet component to reduce the core loss and copper loss. The integrated magnet component comprises three inductors. Individual windings forming the three inductors are wound on a single core, and each of the inductors has small inductance and a small number of turns. The windings of two of the inductors are connected with each other and wound in opposite directions (i.e., interconnected in an “oppositely-wound configuration”).
The fundamental boost DC/DC converter of FIG. 37 employing the single coil 101 requires a great-size, great-weight core in order to achieve sufficient voltage boosting while preventing magnetic saturation of the coil 101. Such a great-size, great-weight core has been a significant hindrance to reduction in the overall size and weight of the DC/DC converter.